TN 295 



No. 9005 










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IC 


9005 



Bureau of Mines Information Circular/1985 



Dust Control in Bag-Filling Operations 



By Jon C. Volkwein and Richard D. Gaynor 




UNITED STATES DEPARTMENT OF THE INTERIOR 



75! 

%NES 75TH AX^ 



Information Circular 9005 

n 



Dust Control in Bag-Filling Operations 



By Jon C. Volkwein and Richard D. Gaynor 




UNITED STATES DEPARTMENT OF THE INTERIOR 
Donald Paul Hodel, Secretary 

BUREAU OF MINES 
Robert C. Horton, Director 



^ 






Library of Congress Cataloging in Publication Data: 




Volkwein, J. C. (Jon C.) 

Dust control in bag-filling operations. 

(Information circular ; 9005) 

Bibliography: p. 21. 

Supt. of Docs, no.: I 28.27:9005. 

1. Mineral industries— Dust control. 2. Bagging— Dust control. I. 
Gaynor, Richard D. II. Title. III. Series: Information circular (United 
States. Bureau of Mines) ; 9005. 



-TN29§,«4- [TH7697.M56] 622s [622\8] 84-600187 



CONTENTS 

Page 

Abs tract 1 

Introduction 2 

Acknowledgments 3 

Ventilation 4 

Hardware design 9 

Water use. 15 

Work practices and housekeeping 18 

Summary 20 

References 21 

ILLUSTRATIONS 

1. Sulfur hexafluoride testing of ventilation hoods 5 

2. Wear-resistant skirting design 6 

3. Laboratory model of overhead air supplied island (OASIS) 8 

4. Typical rooster tail during bag filling 9 

5. Elliptical nozzle on bag-filling machine 10 

6 . New nozzle for bag filling 11 

7. RAM strip chart showing dust levels using new nozzle and conventional 

nozzle 12 

8. Sequence of machine operation showing effect of cleaning-time delay 13 

9. New leakproof polyethylene valve 14 

10. Effect of charged-water sprays on dust concentration in dust box 17 

11. Effect of broom sweeping on bag-machine operator's exposure 18 

12. Stacks to remove dust from plant dust-collector discharge 20 

TABLES 

1. Representative size distributions of whole-grain and ground silica 

products 3 

2. Area background respirable alpha-quartz (RAQ) levels versus area samplers 

at work station 4 

3. Laboratory results from using the OASIS in mills 8 

4. Average RAM results during bagging of 325-mesh ground silica ... 12 

5. Field time study of new self -cleaning nozzle...., 13 

6. Operator's record of bag breakage using free-dried and natural kraft paper 

bags 15 

7. Effect of water on flowability and dustiness of 325-mesh ground silica.... 15 

8. Dust reductions as foam is mixed at transfer points 16 

9 . Effect of cleanup on dust samples 18 





UNIT OF MEASURE ABBREVIATIONS 


USED IN 


THIS REPORT 


cfm 


cubic foot per minute 


mg/m 3 


milligram per cubic 
meter 


ft 


foot 










ym 


micrometer 


f t/min 


foot per minute 










pet 


percent 


gal 


gallon 










Pt 


pint 


h 


hour 










s 


second 


in 


inch 










SCFM 


standard cubic feet 


lb 


pound 




per minute 


L/mln 


liter per minute 


wt pet 


weight percent 


Mg 


microgram 







DUST CONTROL IN BAG-FILLING OPERATIONS 

By Jon C. Volkwein and Richard D. Gaynor 



ABSTRACT 

The Bureau of Mines and many member companies of the Industrial Sand 
Association have been working in several areas to reduce personal expo- 
sure to respirable dust. Areas investigated include ventilation, modi- 
fication of hardware, moisture addition to the product, and improved 
housekeeping practices. 

Ventilation systems have been devised and shown to contain 100 pet of 
a tracer gas released within the hood. An industrial clean air island 
has been developed and shown to reduce dust levels 90 pet at the opera- 
tor's position. 

Modification of hardware on a bag filling machine has been demon- 
strated to reduce dust 83 pet at the machine operator's lapel. This 
particular development has been termed a significant advancement in the 
state-of-the-art of bag filling by industry representatives. 

Addition of moisture to dried product materials has been tested with 
results showing dust reductions of 90 pet are possible. Control of the 
moisture addition method still needs refinement and costs may be 
prohibitive. 

Dust sampling results clearly show the benefits of housekeeping prac- 
tices such as building cleaning and vacuum sweeping. Rotation of work 
schedules is also effective for reducing personal exposure. 



'Physical scientist, Pittsburgh Research Center, Bureau of Mines, Pittsburgh, PA. 
2 Director of Engineering, National Industrial Sand Association, Silver Spring, MD, 



INTRODUCTION 



Early it was recognized that better 
dust control procedures were needed 
around industrial sand bagging oper- 
ations. Starting in 1975 the Bureau of 
Mines entered into a series of coopera- 
tive research agreements with members of 
the National Industrial Sand Association 
(NISA) to improve dust control procedures 
associated with the operation of 
fluidized bagging machines and the con- 
veying, handling, and loading of bagged 
industrial sand products. The early 
cooperative work contributed to the de- 
velopment of the NISA handbook on "Guid- 
ance and Solutions to Reducing Respirable 
Dust Levels in the Bagging of Whole Grain 
Silica Products" (l). 3 

Solutions for bagging fine milled or 
ground products were much more difficult. 
Ground silica sometimes referred to as 
silica flour), is predominately finer 
than the No. 200 sieve, while whole-grain 
products are predominately coarser than 
the No. 200 sieve. Table 1 gives repre- 
sentative ranges in particle size distri- 
bution of whole-grain and ground silica 
products. The industrial sand industry 
is primarily engaged in operating sand 
pits and dredges and in washing, screen- 
ing, and otherwise preparing sand for 
uses other than construction, such as 
glassmaking, molding, and abrasives (2^). 
The hardness of the sand or sandstone 
deposit will determine the method of min- 
ing. A loose unconsolidated sand or 
sandstone can be mined by hydraulic moni- 
toring and pumping in slurry form to a 
wet processing plant. Hard, consolidated 
sandstones may require conventional meth- 
ods of drilling, blasting, loading, and 
hauling to the processing plant. This 
material may require one or two stages of 
crushing to prepare it for wet process- 
ing. Wet processing is often required to 
clean the material and remove clay and 
other material finer than the No. 140 
sieve. Most of the slimes and other 
fines are removed by hydraulic classifi- 
cation with the fines pumped to a 

•^Underlined numbers in parentheses re- 
fer to items in the list of references at 
the end of this report. 



tailings pond to be wasted. Oversize 
material is removed by wet screening. If 
the feed material to the wet-processing 
area is from a hard, consolidated sand- 
stone, wet rod milling in closed circuit 
with screens, is added at the head of the 
wet-process circuit to reduce the materi- 
al to sand-grain size and remove objec- 
tionable oversize material. In many 
cases , in order to meet rigid physical 
and chemical customer specifications, it 
is necessary to provide further benefici- 
ation processes prior to drying for elim- 
ination of deleterious material. 

Material produced meeting the proper 
screen sizing must be dried prior to 
shipment. Drying is generally accom- 
plished by either rotary dryer or fluid- 
bed dryer using natural gas, fuel oil, or 
propane as fuel. Once sand has been 
dried, it may be sold directly or used as 
feed stock for the grinding process. 

Dry material meeting the required phys- 
ical and chemical limits can then be pro- 
cessed into ground silica products. 
Grinding is accomplished in pebble mills 
lined with silica block. Grinding media 
are generally composed of silica pebbles 
or heavy-density ceramic spheres. Grind- 
ing of the material in the pebble mills 
is in closed circuit with air classifiers 
in order to produce a specified size 
product. The ground silica product is 
then transported to storage prior to 
loading in bulk containers or as bagged 
material. 

Because of the fineness of ground sil- 
ica, exposure to respirable silica tends 
to be excessive when the material is 
bagged. Often, if not generally, expo- 
sure to respirable quartz (or silica) 
approaches or exceeds the current MSHA 
permissible exposure limit (PEL) of 0.1 
mg/m 3 . For this reason, workers general- 
ly are required to wear respirators at 
their work stations when bagging ground 
silica, stacking, handling, or loading 
the bagged materials. The principal 
objective of the studies was to devise 
methods whereby it would be possible to 
work without the encumbrance of 



TABLE 1. - Representative size distributions of whole-grain 
and ground silica products 



Size designation 1 


Fraction passing indicated sieve, wt pet 


Sieve No. 


Opening, 

ym 


Whole grain 2 


Ground 




Coarse 


Fine 


Coarse 


Medium 


Fine 


8 

40 

50 

140 


2,360 

1,700 

850 

600 

425 

300 

212 

150 

106 

75 

45 


100 

95 

25 

2 

1 

NAp 

NAp 

NAp 

NAp 

NAp 

NAp 


NAp 

NAp 

NAp 

NAp 

99 

79 

45 

16 

3 

1 

NAp 


NAp 

NAp 

NAp 

NAp 

NAp 

NAp 

NAp 

NAp 

62 

51 

33 


NAp 

NAp 

NAp 

NAp 

NAp 

NAp 

NAp 

NAp 

97 

94 

77 


NAp 
NAp 
NAp 
NAp 
NAp 
NAp 
NAp 
NAp 
100 
99 
98 



NAp Not applicable. 

X ASTM E 11-81, Specification for 
Testing Purposes. 

2 Coarse (No. 8 by No. 30) — AFS Grai 
(No. 40 by No. 200) — AFS Grain Finene 



Wire-Cloth Sieves for 

n Fineness No. 12; fine 
ss No. 64. 



respirators. The paper summarizes ef- 
forts made by both government and in- 
dustry to control dust in bag-filling 
operations. Broadly, the engineering 
controls considered include ventilation, 
modification of the fluidized packing 
machine, improvements in bag design and 



construction, and the use of moisture, 
foam, steam, or charged spray. Also con- 
sidered are the effects of administrative 
controls and good housekeeping, which are 
considered extremely important in reduc- 
ing silica exposures. 



ACKNOWLEDGMENTS 



Special credit is due the members of 
the NISA Committee on Engineering and 
Technology who have guided, advised, and 
performed much of the work reported here. 
None of this would have been possible 
without the support and cooperation of 
the companies they represent. 

James P. Snider, Chairman, Central Silica 

Co. 
John J. Bryant, George W. Bryant Core 

Sands, Inc. 
Timothy P. Bryant, George W. Bryant Core 

Sands, Inc. 
Paul V. Castellini, Jesse S. Morie & Son, 

Inc. 
Douglas Gerner, Walter C. Best, Inc. 
Robert L. Hawvermale, Pennsylvania Glass 

Sand Corp. 
Thomas M. Kilroy, Jr., Unimin Corp. 
E. Stuart Kirkpatrick, Pennsylvania Glass 

Sand Corp. 



Richard A. Kruback, Whitehead Brothers 

Co. 
Richard Reed, Martin Marietta Industrial 

Sand Div. 
Earl R. White, Ottawa Silica Co. 
Walter 0. Wright, South River Sand Co. 
Andrew B. Cecala, Bureau of Mines 

Special recognition is extended to 
the following members of NISA and the 
Bureau of Mines who took an early, ac- 
tive interest in the problems of the 
silica-bagging industry and whose vision 
and leadership contributed to the success 
of this paper: Arnold A. Alekna (de- 
ceased) of Martin Marietta Laboratories, 
Lester C. Richards (retired) of Ottawa 
Silica Co. , and Thomas E. Rosendahl of 
RET Consulting (formerly with Bureau of 
Mines) . 



VENTILATION 



An adequate exhaust ventilation system 
is essential for good dust control around 
bag-filling operations. To be most ef- 
fective, hoods should be used to contain 
and direct the exhaust airflows. General 
guidelines for hood and duct designs 
can be found in the Industrial Ventila- 
tion Manual published by the American 
Conference of Governmental Industrial 
Hygienists (3). 

The Bureau designed a new ventilation 
hood specifically for nozzle-type bag- 
filling machines (4^ . The ability of 
this hood to capture a contaminant gen- 
erated inside the hood was measured using 
a tracer gas technique (_5-6) . The tracer 
gas, sulfur hexaf luoride, was released at 
a steady rate inside the hood near the 
bag-and-nozzle interface. Bag filling 
proceeded normally, and air samples were 
taken at the operator's breathing zone 
and at the front of the ventilation hood. 
Figure 1 shows the tracer gas test equip- 
ment, a sample being taken, and the ven- 
tilation hoods. 

Results of this testing showed that (1) 
The intake face velocity of these hoods 
should not be less than 200 ft/min; (2) 
natural or mechanical airflows should not 
be directed toward the hoods, (3) the 
nozzle area of the hood must be correctly 
sealed, and (4) hoods, ducts, and collec- 
tors must be maintained. When working as 
designed, these ventilation hoods cap- 
tured 100 pet of the sulfur hexafluoride 
(SF 6 ) tracer gas released inside the 
hood. 

Unless conveyor systems that support 
the bagging operations are properly en- 
closed and ventilated, they may add to 
the dust exposure of workers. Under a 
recent Bureau of Mines contract, Martin 
Marietta Laboratories (7) studied ef- 
fective enclosure systems suitable for 
conveyors that feed product to bag- 
filling machines. Figure 2 illustrates a 



wear-resistant skirting design that of- 
fers a longer lasting, more effective 
seal to the edge of the belt. Other de- 
sign features important for dust control 
include rock boxes, chutes to reduce free 
fall, impact idlers, enclosure size, ven- 
tilation volumes, and various seals. 

The Martin Marietta study showed that 
when good design principles are used, the 
exhaust air volume requirements for 
transfer points can be minimized. Re- 
sults obtained using good engineering 
designs and exhaust volume estimates have 
shown that dust from a conveyor-belt 
transfer point could be reduced by 
95 pet. 

Large volumes of exhaust ventilation 
air require that an equal volume of make- 
up air be supplied to the area. The 
quality of the background air then be- 
comes important when evaluating worker 
dust exposure. Emission-rate-sampling 
data show that the background dust levels 
in both whole-grain and ground silica 
bagging operations can contribute to more 
than one-half of the total respirable 
dust exposure of a worker. For example, 
table 2 shows area dust measurements in 
the intake air and at the work station of 
a ground-silica bagging machine, prior to 
installation of dust controls. On the 
first day, 75 yg of quartz was in the in- 
take air. The worker station was exposed 
to 118 yg of respirable alpha-quartz. On 
the following days, the intake levels of 
quartz were more than one-half of the 
quartz present at the worker station. 

TABLE 2. - Area background respirable 
alpha-quartz (RAQ) levels versus 
area samplers at work station 



Day 1 
Day 2 
Day 3 



RAQ level, mg/m 3 



Intake 



0.073 
.313 
.246 



Work station 



0.118 
.607 
.399 




FIGURE 1. - Sulfur hexafluoride testing of ventilation hoods. 




FIGURE 2. - Wear-resistant skirting design. 



Clearly, improvement of the exhaust 
ventilation system on the machine itself 
would not achieve compliance for two of 
the three days at this location. Even if 
the machine were dust-free, it would 
still be difficult to comply with the 
standards for alpha-quartz because the 
background dust levels were so high. 
This is not an isolated case. Signifi- 
cant levels of background dust have been 
encountered in most whole-grain and 
ground silica mills. This dust entered 
the work area from locations and opera- 
tions outside of the bagging areas. 

Factors that can account for high back- 
ground dust levels include the following: 

• Meteorological conditions (amount of 
precipitation and wind speed and 
direction) , which can directly ef- 
fect fugitive dust sources, such as 
haulage roads and stockpiles outside 
the building. 

• The proximity of other dust opera- 
tions where personnel do not normal- 
ly operate or are in controlled 
booths, such as the grinding mills 
found in plants. Open transfer 
points within the same building or 
bulk loading facilities immediately 
adjacent to the building and poor 
housekeeping also contribute to 
background dust levels. 

• Location of exhaust stacks, which 
may be such that fans used for ven- 
tilation recirculate dust into the 
building and contribute to the back- 
ground dust problem. Particulate 
from stacks that comply with envi- 
ronmental standards can recirculate 
a high fraction of respirable dust 
and become a significant background 
source. 

• The overall topography of the plant 
site. 



background dust may involve three basic 
approaches: 

• Defining the specific background 
source and controlling it. 

• Isolating the bagging operations 
from possible background dust 
sources and/or locating the dust 
sources away from the bagging 
operations. 

• Providing the operator with a source 
of clean makeup air, either by 
locating the air inlet away from the 
dust source or by filtering. The 
ACGIH Ventilation Handbook (3) 
provides guidelines for makeup air 
systems. 

A filtration approach was taken at a 
whole-grain bagging operation where dust 
from an adjacent bulk-loading facility 
was being drawn into the bagging building 
by the exhaust ventilation system of the 
bagging machine. The company, in consul- 
tation with the Bureau of Mines , engi- 
neered and installed ventilation hoods, a 
nozzle cleanout system, and a makeup air 
ventilation system. Respirable dust lev- 
els at the new installation were in com- 
pliance as measured by the Mine Safety 
and Health Administration (MSHA) , U.S. 
Department of Labor. Experience has 
shown that all three dust control methods 
must be maintained to stay in compliance. 

Another approach that has been tried to 
control background dust levels involved 
the ionization of air. In theory, air 
ionization charges dust particles so that 
they are electrostatically attracted to 
an oppositely charged surface. Two sep- 
arate industrial sand plants have ex- 
perimented with the Apsee 4 air ionization 
system but have reported no dust re- 
duction with its use. At one plant, the 
manufacturer suggested that a second unit 
was needed to deliver an additional 



Because of low threshold limit val- 
ues, background dust levels are very 
important. Protecting the operators from 



^Reference to specific equipment does 
not imply endorsement by the Bureau of 
Mines. 



volume of ionized air, but this, too, was 
unsuccessful. The plant personnel con- 
cluded that the exhaust ventilation sys- 
tem in the plant removed the ionized air 
before it had a chance to become 
effective. 

Since it can be expensive to supply 
large volumes of conditioned makeup air, 
the Bureau of Mines has been experiment- 
ing with an overhead air supplied island 
(OASIS) for mills. This device supplies 
5,000 cfm of clean air to the workers' 
breathing zone and may reduce the volume 
of clean makeup air normally required. 
The concept is based on a canopy air 
curtain (8-9) developed earlier for 
continuous-mining machines. Figure 3 
shows a working laboratory model of the 
OASIS makeup air system. The access cov- 
ers are removed to show the fan and mo- 
tor, which bring air into the top filtra- 
tion panels through the fan and out 
through the final panel filters through 
the bottom of the unit. In laboratory 



tests, respirable dust concentrations 
were measured outside the canopy air cur- 
tain system and 2 ft beneath the canopy, 
using a real-time aerosol monitor. Four 
8-h tests were conducted: two at high 
concentrations and two at low concentra- 
tions. Results in table 3 show that the 
average efficiency was 89 pet for the 
unit. Field tests on this system are 
scheduled. 

TABLE 3. - Laboratory results 
from using the OASIS in mills 



Dust concentration, 

mg/m 3 


Dust 
reduction, 


Outside 


Inside 


pet 


6.50 

3.60 

.53 

.52 


0.63 
.48 
.08 
.02 


90 

87 

*83 

95 



*New filter (initial efficien- 
cy of filter is expected to be 



low until dust 
filter) . 



cake builds on 




FIGURE 3. - Laboratory model of overhead air supplied island (OASIS). 



The area where bagged material is load- 
ed into vehicles is a final location 
where good ventilation is required. The 
last step of the bagging operation is 
often the stacking of bags inside a 
closed vehicle. Dust adhering to the bag 
surface, leaking bag valves, and broken 
bags all can contribute to dust genera- 
tion inside the vehicle. Without proper 
ventilation, these sources are cumulative 
and can result in high exposures. In an 
attempt to provide ventilation inside a 
van-type trailer, one company suspended a 



section of flexible tubing into the 
trailer and blew about 3,000 cfm of air 
into the truck. No respirable dust data 
were taken, but workers and officials 
claimed that the high-velocity jet aero- 
solized more dust from surfaces than 
it removed from the vehicle. Use of 
the system was discontinued. The Bureau 
plans to conduct some laboratory tests 
to study how to introduce air into a 
closed vehicle without creating excessive 
turbulence. 



HARDWARE DESIGN 



Good hardware design that prevents dust 
from being generated is preferable to re- 
medial attempts to capture the dust from 
the air. Problem areas in bag filling, 
such as blowback and "rooster tails" 
(fig. 4), are very difficult to control 
with exhaust ventilation. 



Early attempts to control the blowback 
(venting of air and product from the bag 
valve-fill tube interface) of fluidized 
product through the bag valve involved 
the use of inflatable bladders, tapered 
sleeves, and reshaping the fill nozzle. 
The inflatable bladder approach is 




FIGURE 4. - Typical rooster tail during bag filling. 



10 



effective in preventing blowback, but few 
operators have been able to maintain the 
fragile bladders. Round tapered sleeves 
and elliptical filling nozzles, which 



better approximate the shape of the bag 
valve, are much more durable (fig. 5). 
However, these are not totally effective 
in controlling blowback. 




FIGURE 5. - Elliptical nozzle on bag-filling machine. 



11 



The rooster tail that results from the 
product and fluldized air pressure in the 
bag is an extremely difficult problem to 
solve. Exhaust ventilation is not effec- 
tive in capturing this release of mate- 
rial because it is thrown outside of the 
ventilation range. In whole-grain sand, 
a simple blast of compressed air in the 
nozzle after the bag was filled was suf- 
ficient to prevent the rooster tail (_1_ ) . 
However, this approach has been ineffec- 
tive with finer products such as ground 
silica. 

St. Regis Packaging Machinery Group, 
the Bureau, and an industrial sand plant 
experimented with a novel three-way ro- 
tary valve manufactured by St. Regis 
(10). The valve provided a vent for ex- 
cess product and air at the end of the 
fill cycle. However, products still re- 
mained in the tip of the fill tube and 
spilled as the bag left the machine. 
This spillage was successfully controlled 
by using a short jet of compressed air at 
the tube-and-bag interface while simul- 
taneously venting the bag. Technically, 
this system is effective in eliminating 
the rooster tail, but the rotary valve, 
which is expensive, would have insuffi- 
cient durability in contact with abrasive 
products. An attempt to develop an al- 
ternative valve was not successful. 

One industrial sand company has devel- 
oped a "duck bill" filling tube designed 
to reduce or eliminate the rooster tail 
and blowback. The patent is pending on 
this development and full details are un- 
available. The filling tube was reported 
to be only partially successful, since 
the rooster tail and blowback were not 
eliminated. 

Champion International Corp. has been 
experimenting with a new fill tube design 
to solve some of the bagging problems. 
The company's purge and vacuum system 
will reduce product loss due to spillage 
during bag discharge, and complement 
a new commercially available siftproof 
bag valve, which will be discussed 
later. Although the new fill tube is not 
specifically designed to reduce dust 
generation, any reduction in spillage 



should reduce dust levels. Details on 
this development will be available when 
system testing is completed. 

The Bureau of Mines, through a contract 
with Foster-Miller, Inc. , has devised a 
bag-filling nozzle that reduces blowback 
and eliminates the rooster tail (11) . 
Industrial representatives consider this 
new nozzle to be a significant advance in 
the state-of-the-art of bag filling. The 
system (fig. 6) has three unique fea- 
tures: First, a new bag clamp applies 
uniform pressure for about 270° around 
the top of the bag valve. This greatly 
reduces blowback and allows excess air to 
vent through the bottom of the bag valve. 
Second, another tube around the fill tube 
evacuates product material in the nozzle- 
and-valve area after the bag is full. 
This relieves the fluidizing air pressure 
within the bag and eliminates the rooster 
tail. Third, the exhaust remains on for 
approximately 2s as the bag discharges 
from the machine and vacuums the bag 
valve clean. 




FIGURE 6. - New nozzle for bag filling. 



12 



The new system was retrofitted on a 
four-tube ground silica-packing machine. 
To determine the dust reduction effec- 
tiveness of the new hardware before and 
after installation, dust surveys were 
conducted using real-time aerosol moni- 
tors (RAM) and gravimetric samplers. 
Each product grade was sampled on an in- 
dividual basis (since finer grained prod- 
ucts are generally dustier). The RAM in- 
struments were especially useful for this 
purpose. 



Figure 7 shows a section of RAM strip 
chart for a typical test. This compares 
dust levels during the bagging of 325- 
raesh silica with the old nozzle system 
and the dust levels after installation of 
the new nozzle system. Dust measurements 
were taken at the operator's lapel. The 
average reduction measured for 325-mesh 
product size was 83 pet. Equivalent dust 
reductions were measured at other loca- 
tions and results are shown in table 4. 



TABLE 4. - Average RAM results during bagging of 325-mesh ground silica 



Location 


Normal 
levels , 
mg/m 3 


Levels with 

new nozzle, 

mg/m 3 


Difference, 
pet 




>200.00 1 
.33 
.29 
.42 
.32 


21.87 
.13 
.06 
.07 

.07 


>89 


Bag conveyor to dock conveyor transfer point 
Bag room air intake (from loading dock area) 


61 
79 
83 

78 



*RAM off scale. 



ro 



2.0 



E 



55 

en 



LU 

o 

o 
o 

h- 

C/) 

Z> 
Q 



Operator's lapel, 325 mesh 



Conventional system. 




FIGURE 7. - RAM strip chart showing dust levels using new nozzle and conventional nozzle. 

''Dashed line is threshold limit value.) 



13 



The new filling hardware does use a 
time delay to clean the fill nozzle. The 
addition of a 7-s time delay did not have 
a significant effect on productivity. 
Figure 8 shows that an extra 7-s delay 
per machine is actually distributed over 
the fill time of four machines: 



Delay per bag = 



Time delay per machine 
Number of machines 

^ = 1.75 s. 



Actual timing of the old and new hardware 
(table 5) confirms that the new hardware 
adds about 1.5 s to the bag-filling cycle 
on a four-station machine. Other produc- 
tivity considerations that were not quan- 
tified include reduced cleanup around the 
machine, reduced product loss, less bag- 
house dust loading, cleaner bags, and 
cleaner workers. Foster-Miller, Inc., is 
now marketing this new hardware and has 
applied for a patent. As testing of the 
system continues, some areas of wear and 
maintenance of equipment are being noted. 
A second unit recently installed correct- 
ed some of the inherent problems encoun- 
tered in the first unit. The performance 
and operation of the Foster-Miller bag- 
ging system will continue to be improved. 

TABLE 5. - Field time study of new 
self -cleaning nozzle 









Additional 


Product 


Actual time 


Additional 


time to 


grade, 


to fill 50 


time per 


load truck 


mesh 


bags , sec 


bag, sec 


with 480 
bags , min 


120.... 


70 


1.4 


11:12 


180.... 


83 


1.6 


13:17 


325.... 


76 


1.5 


12:00 



Once product material is packaged, the 
objective is to prevent spillage and bag 
breakage. The design of the bag valve 
plays an important role in reducing 
spillage during bag handling. The basic 
bag valve consists of a 3- to 5-in tube 
of kraft paper inserted into the bag. 
This tube or sleeve receives the nozzle 
during the filling cycle and collapses 
when the bag falls onto the discharge 
conveyor. Product material is frequently 



Time to fill 4 bags with 
cleaning delay 

Time to fill 4 bags, 
no cleaning 

Total time added to cycle 

i 




TIME, arbitrary units 

FIGURE 8. - Sequence of machine operation 
showing effect of cleaning-time delay. 

trapped within this sleeve and, as the 
stiff kraft paper flexes during handling, 
product dribbles from the bag valve. A 
better bag valve uses a thin polyethylene 
tube in place of the kraft paper. This 
design still traps product within the 
tube, but the flexible plastic is less 
prone to flexing open and spillage is re- 
duced, although not eliminated. 

Foster Miller attempted to develop a 
mechanical bag valve seal that was com- 
patable with existing bag-manufacturing 
equipment (11). Their idea used a two- 
component sleeve where a stiff preformed 
plastic acted like a spring to draw a 
more flexible plastic sleeve closed. In 
shipment to the plant , it was found that 
the preformed plastic sleeve lost its 
shape and did not perform as designed. 

Champion International Corp. recently 
introduced a unique leakproof poly- 
ethylene bag valve (called "Sift Proof 
Valve"), which consists of a sealed tube 
with a slit facing the bottom of the bag 
(fig. 9). During filling, the polyeth- 
ylene valve opens to direct the flow of 
product downward to the bottom of the 
bag. The force of the product flow 
stretches the polyethylene during fill- 
ing. The stretched polyethylene then 
overlaps after the bag leaves the ma- 
chine. Preliminary reports suggest 
that the sleeve does help to reduce 
spillage. A patent has been issued on 
this development. 



14 







, v ;■ ■.,,■,■..:■.■.;. 







».J? % » . « ■■ ■ ■ ■ ■f%&j^*#**i^J^'*#^* ^ iii )to'i wt sfi&mm 



\\ 



~ ' mL- 'fxnw- 




FIGURE 9. - New leakproof polyethylene valve. 



Bag breakage can be a substantial 
source of respirable dust, especially 
with fine product sizes. Use of stronger 
bags can reduce breakage and customer 
complaints while increasing productivity. 
The records of one company (table 6) in- 
dicate that the frequency of broken bags 
can be reduced when a new three-ply , 50- 
lb tension-free-dried paper (also called 
"free-dried" paper) is used instead of a 



standard three-ply, 60-lb natural kraft 
paper. Free-dried paper gets its in- 
creased strength and resiliency from the 
fact that it is allowed to stretch in two 
directions rather than one when being 
manufactured. Bag breakage was reduced 
by a factor of 12. Based on the record, 
this industrial sand plant now specifies 
this type paper and finds the cost to be 
comparable to the previous type. 



15 



TABLE 6. - Operators record of bag breakage using free-dried and 
natural kraft paper bags 



Paper 


Product 


Number of 
bags loaded 


Number of 
bags broken 


Breakage , 
pet 




Sand 


8,540 
7,080 
4,420 
2,250 
2-530 


5 
30 

1 
15 
29 


0.06 


Natural kraft.. 
Free-dried. .... 
Natural kraft.. 


• • • CLO •••••••• 

• ••QO»* •••••• 

Ground silica 


.4 
.02 
.7 
.4 



WATER USE 



Use of water in a bagged, dry product 
to control dust has not been widely prac- 
ticed in the past. However, its use has 
been reexamined as a means of reducing 
worker exposure and simplifying complex 
engineering and administrative controls 
that are often required. Naturally, wa- 
ter cannot be used (1) with materials 
that react chemically, (2) or in materi- 
als where its use produces handling prob- 
lems , or (3) where it is unacceptable to 
the customer. However, there appear to 
be a number of applications where wa- 
ter can be of benefit in reducing dust 
levels. 

Water can control dust two distinct 
ways. First is suppression, which works 
by wetting product materials and causing 
the dust to adhere to the product or 
other dust particles; this prevents it 
from becoeing airborne. The second is 
airborne capture, in which water droplets 
collide with dust particles in the air, 
increasing their size and causing them to 
drop from the airstream. Most of the 
following experiments using water were 
designed to suppress dust rather than to 
capture it from the air. 

Early work conducted in an industrial 
laboratory studied the effects on flowa- 
bility and dust suppression by adding 
various quantities of water to 325-mesh 
silica. All samples were uniformly 
blended in a zig-zag blender (continuous 
feed). The flowability of the silica was 
then graded on a scale of 1 to 5, with 5 
being optimum. Samples were then dropped 
from a height of 4 ft onto a paper placed 
on a black surface. The resulting dust 
cloud was judged by a three-person panel 



and graded 1 to 5, with 5 being the least 
dusty. The results of this study are 
summarized in table 7. From a material- 
handling viewpoint, no more than 1 to 2 
pet moisture should be added. It appears 
that some moisture can help the flowabil- 
ity of the sand by removing the static 
charge found in the totally dry sand. 
Dust suppression did not occur, however, 
until moisture rose over 3 pet. 

TABLE 7. - Effect of water on flowability 
and dustiness of 325-mesh ground silica 



Sam- 


Added water, 


Flowability, 


Dustiness, 


ple 


wt pet 


grade 1 


grade 2 


1 





1 


1 


2 


1 


5 


3 


3 


2 


4 


3 


4 


3 


2 


5 


5 


4 


1 


5 


6 


5 


1 


5 



trades 1-5; 5 is best flow. 
2 Grades 1-5; 5 is least dustiness. 

The recent Bureau of Mines contract 
with Martin Marietta Laboratories (7) 
studied wet collection-and-suppression 
systems on a roll crusher to belt trans- 
fer point in a crushed limestone plant. 
Although this plant was not producing 
dried mineral product, the results are 
applicable to dried minerals operation. 
Martin Marietta measured the dust control 
achieved using air-atomized water sprays 
at the crusher discharge and a simple 
hydraulic water spray at the crusher in- 
let. The results showed that the water 
added as a mist, after crushing, was 55 
to 65 pet effective in collecting respi- 
rable size dust. Adding water to the ore 
before crushing and allowing it to mix 



16 



thoroughly within the crusher suppressed 
respirable dust 70 to 80 pet. Using both 
systems together the effectiveness in- 
creased 80 to 95 pet. 

Two facts of interest from this study 
are that mixing water with the ore in the 
crusher provides good dust suppression, 
and that even the best airborne dust cap- 
ture spray (air atomizing)^ did not have 
sufficient contact time with the dust to 
be most effective. 

In the past, researchers have tried 
various types of foam to suppress dust in 
coal mines ( 13-14) , spraying foam on top 
of the coal much like water spray. The 
results showed that foam was not much 
better than a good water spray under the 
test conditions. New interest in foam 
systems for minimal moisture addition to 
dried products, combined with manufactur- 
ers claims of superior dust control using 
foam, led the Bureau to test the effec- 
tiveness of foam at two industrial sand 
plants. These studies differ from previ- 
ous work in that the foam was thoroughly 
mixed into dried product materials. 

Results in table 8 show that as foam 
is mixed with 30-mesh glass sand from a 
transfer point to transfer point, its 
dust suppression effectiveness increases. 
Additional testing using more foam showed 

"For details on the comparison of spray 
nozzle effectiveness for airborne cap- 
ture, see Bureau of Mines Technology News 
150 (12) . 



dust reduction of 80 to 90 pet on three 
separate occasions at two different 
plants ( 15 ) that process whole grain 
sands. Limited work has been conducted 
on the applicability of foam in ground 
silicas. Although visual tests have in- 
dicated that foam has good potential, no 
quantitative studies have been conducted. 
Before foam can be considered as a dust 
control for whole-grain or ground silica, 
the following must be considered: 

• The foaming surfactant must be com- 
patible with the end uses of the 
product. Ultrapure grades of cer- 
tain products cannot tolerate even a 
few parts per million of surfactant. 

• Foam generators must be easy to con- 
trol and regulate. This is espe- 
cially critical if minimum moisture 
levels on the order of a few tenths 
of a percent are to be maintained. 

• Evaporation reduces the effective- 
ness of treatment. This is more 
likely to be a problem at high prod- 
uct temperatures. 

• Foam is relatively expensive. The 
average amount of surfactant per ton 
of sand treated was 0.012 gal. At a 
surfactant cost of $7.25/gal, the 
cost to treat each ton of sand is 
$0.09 (exclusive of capital and pow- 
er costs). Depending on usage, this 
number can vary from a low of $0.04/ 
ton to a high of $0.20/ton. 



TABLE 8. - Dust reductions as foam is mixed at transfer points 



Location and condition 


Av . of 4 
filters, mg 


Std. dev. of 
4 filters 


Dust 
reduction, 
pet 


Transfer point 1: 


7.41 
5.95 

5.59 
3.76 

6.69 
2.46 


1.83 
.87 

.10 
.05 

.29 

.10 


1 ,o -, 




19.7 


Transfer point 2: 


J 

1 oo n 




32.7 


Bulk loadout: 


J 

"1 « o 




65.3 



17 



Foam appears to work by uniformly add- 
ing small amounts of water to the dried 
product material. The surfactant used to 
generate the foam has no measurable ef- 
fect on dust suppression, but is only 
necessary to increase the surface area of 
the water, thus distributing it uniformly 
into the product. 

Studies showed that the important dust- 
suppressing properties of foam are the 
addition of moisture, large water surface 
area, and good mixing with the dusty ma- 
terials. Since none of these properties 
is unique to foam, the Bureau decided to 
try fine mist sprays and steam for dust 
suppression ( 16 ) . Previous work by the 
Bureau with steam had focused on airborne 
collection rather than on suppression 
(17). The new experiments were conduct- 
ed at two industrial sand plants where 
an engine-cleaning-type generator made 
steam, which was added whole grain sand 
and mixed at a conveyor belt transfer 
point. Results showed that for an equiv- 
alent amount of water, steam was twice as 
effective as water in suppressing dust. 
Even with poor mixing and the preliminary 
nature of these tests, average dust re- 
ductions using steam were about 65 pet 
when adding 0.22 wt pet moisture. Steam 
will not contaminate the product material 
and is easily controlled; however, de- 
tailed engineering will be required to 
design a mixing system to prevent conden- 
sation and material buildup. The heat 
needed to make the steam also can be ex- 
pensive; estimates range from $0.06 to 
$0.08/ton of material. 

A use of water in bagging operations 
was tried at an industrial sand plant, 
where a company-designed air-atomized 
spray system was used to wet the nozzle 
area of the bag during filling and trans- 
port. The company also experimented with 
injecting water into a ground silica 
product as it flowed into the bag. The 
Bureau monitored the resulting dust re- 
ductions (18) . Generally, these methods 
reduced dust by about 50 pet, but of 
equal significance was the fact that wet- 
ting the bag valve during filling reduced 
dust levels by 50 pet inside rail cars 



where the bags were being palletized. 
Injection of water into the product was 
discontinued after the experiment because 
of customer complaints regarding frozen 
shipments of sand. Currently, the out- 
side surface of the bag is still being 
wetted. 

Foster-Miller, through Bureau of Mines 
contract ( 19 ) , compared the effectiveness 
of using electrically charged water 
sprays and uncharged water sprays for 
airborne capture of dust. Figure 10 
shows that, regardless of the charge or 
type of dust, charged sprays were supe- 
rior to uncharged sprays. Charged water 
sprays were found to — 

• Be the best per unit use of water 
for airborne capture of dust. 



• Require a long residence 
the dust to capture it. 



time with 



o 
o 

o 






UJ 

o 

-z. 
o 
o 

(/> 

CO 
UJ 



o 




I0" 3 

I 2 3 

TIME, min 

FIGURE 10. - Effect of charged-water sprays 
on dust concentration in dust box. 



18 



• Need water free of suspended solids. 

• Not able to be used where static 
electrical sparks are a hazard. 

Using water for dust control for 
dried mineral products is still in the 



experimental stage, but results to 

date indicate that it is potentially 

an effective tool for the dust control 
engineer to consider. 



WORK PRACTICES AND HOUSEKEEPING 



The best engineering technology cannot 
keep plant dust levels low if the work- 
place is not kept clean and orderly. 
In much the same way as a strong safety 
program requires education and support 
from management, a strong housekeeping 
program requires the same commitment. 
The following work and housekeeping 
practices are essential to good dust 
control. 

Removing accumulations of dust from 
floors and ledges in the work area re- 
duces the amount of dust that can become 
airborne through vibration or wind cur- 
rents. The data in table 9 compare 
respirable dust from work area samples 
taken in a screen tower before and after 
cleanup. The cleanup reduces exposures 
by 75 pet of their bef ore-cleanup level. 
In this instance, the exposure is still 
high and would require the use of 
respirators. 



TABLE 9. ■ 
samples 



Effect of cleanup on dust 



Sampling period h. . 

Total respirable 
dust. mg/m 3 . . 

Respirable quartz. .mg/m 3 . . 



Before 


After 


clean- 


clean- 


up 


up 


4.0 


5.5 


6.69 


1.40 


1.46 


0.35 



Use of brooms and shovels to clean up 
spills and broken bags must be avoided 
since large amounts of dust will be 
generated. Figure 11 shows the exposure 
of a bag machine operator when a worker 
on the floor below was using a broom to 
clean up. Vacuum systems represent the 
preferred method of cleanup. Although 
available performance data are limit- 
ed, most vacuums are centrally located 
systems that can service large areas 
throughout a plant. One satisfactory 




4 6 

TIME, min 

FIGURE 11. - Effect of broom sweeping on bag-machine operator's exposure. 



10 



19 



system produces 600 SCFM at 8 in Hg. 
This system is used for general house- 
keeping, and to clean up equipment prior 
to maintenance operations. However, be- 
cause it is slow picking up broken bags 
of material, and a larger system is rec- 
ommended for this application. 

Many new plants are being designed for 
washdown. This design includes proper 
drainage and protection of the electrical 
system. In some cases, plants have been 
able to retrofit existing buildings for 
washdown. In cases where washdown is not 
possible, floors have been mopped or 
sprayed with mineral oil products to help 
capture and retain dust that collects. 

Dust exposures are often high where 
bags are manually handled, stacked, or 
palletized. When pallets are loaded into 
enclosed spaces such as box cars or en- 
closed trailers, exposure can be high. 
Box cars and trailers should be cleaned 
with a vacuum before loading. Spills and 
broken bags must be cleaned up promptly. 
When bags are palletized manually, work- 
ers should place the bags rather than 
throw them. 

Workers' practices in caring for their 
work clothes and personal cleanup after 
work deserve attention. Air hoses must 
not be used since such use resuspends 
dust in the air and is inherently danger- 
ous. Although use of company-supplied 
uniforms is not widespread in the indus- 
trial sand industry, it has been used as 
a part of a program of company relations 
and good housekeeping in some bagging 
industries. 

MSHA requires posting and the use of 
respirators in all work areas where res- 
pirable quartz exposures exceed the per- 
missible exposure level of 0.1 mg/m 3 . By 
rotation of work assignments, it is often 
possible to minimize workers potential 
exposure. Workers bagging or loading 
ground silica might rotate jobs with 
workers bagging whole-grain silica or 
performing other duties. 



An essential ingredient of good house- 
keeping and good work practices is an or- 
ganized program that starts at the high- 
est levels of company management and 
makes each level responsible for the one 
below it. Consequently, the plant mana- 
ger and shift supervisor bear primary 
responsiblility, but they must have the 
support of top management. 

Housekeeping duties must be performed 
on a timely basis. Spills and broken 
bags must be cleaned up right away. De- 
pending upon conditions, other areas may 
need to be cleaned daily, weekly, or even 
monthly. As a part of this regular 
housekeeping, the ventilation system must 
be maintained on a regular preventative 
maintenance schedule. Air velocities at 
pickup points should be checked on a reg- 
ularly scheduled basis just like other 
maintenance checks are made on such items 
as blowers, controls, materials-handling 
equipment, gates, valves, and vehicles. 
Leaks and obstructed ducts must be de- 
tected before system performance suffers. 

The dust in the air around the plant 
constitutes a significant proportion of 
the total dust exposure. Although ground 
silica is 100 pet quartz, the quartz con- 
tent of respirable dust samples taken 
around bagging operations is frequently 
less than 50 pet and sometimes as low as 
10 or 20 pet; the remainder is background 
dust. 

Significant amounts of background dust 
can come from the discharge of ventila- 
tion systems, bag houses, and wet scrub- 
bers. Careful attention to the location 
of ventilation discharges and the dis- 
charge from dust collection equipment can 
help reduce background dust levels. In 
one instance, extending stack heights to 
1-1/3 times the height of adjacent build- 
ings (fig. 12) placed the discharged dust 
above the natural turbulence that is de- 
veloped around and downwind from the 
building, background dust levels were 
significantly reduced. 



20 




FIGURE 12. - Stacks to remove dust from plant dust=col lector discharge. 



Unpaved roads and worked out portions 
of pits are potential sources of back- 
ground dust. Unpaved roads can be 
sprayed with water and/or commercial dust 
treatment compounds. Water is an econom- 
ic solution only on temporary roads. The 
commercial treatment compounds are better 
in areas of moderate rainfall. Paved 



roads and parking areas may require the 
use of an ordinary street sweeper or 
automatic sprinkling system to keep them 
clean. A program of reclamation by 
planting may be used to avoid excessive 
pollution from runoff and will help con- 
trol potential dust during dry periods. 



SUMMARY 



Dust levels in the packaging areas 
of plants that process dried mineral 
products can be difficult to control. 
Three primary dust control techniques 
are available: (1) Ventilation of the 
area (both exhaust and clean makeup air) , 
(2) redesign of hardware to produce 
less dust, and (3) careful addition of 
water (under the proper circumstances). 



Administrative controls can be used to 
reduce a workers time in a dusty area and 
reinsure that the workplace is kept clean 
and orderly. The techniques summarized 
in this paper and described in detail in 
the references cited can be effective in 
reducing dust exposure in the bagging of 
industrial mineral products. 



REFERENCES 



21 



1. National Industrial Sand Associa- 
tion. Guidance and Solutions to Reducing 
Dust Levels in the Bagging of Whole Grain 
Silica Product. Silver Spring, MD, 1977, 
35 pp. 

2. U.S. Office of Management and Bud- 
get. Titles and Descriptions of Indus- 
tries. Part 1 in Standard Industrial 
Classification Manual. 1972, p. 40. 

3. American Conference of Governmen- 
tal Industrial Hygienists. Industrial 
Ventilation. Edwards Brothers, Ann 
Arbor, MI, 16th ed. , 1980, pp. 6-1 to 6- 
42. 

4. U.S. Bureau of Mines. Dust Con- 
trol for Bag Filling Machines. Technol. 
News, No. 54, June 1983, 2 pp. 

5. Thimons , E. D. , R. J. Bielicki, 
and F. N. Kissell. Using Sulfur Hexa- 
fluoride as a Gaseous Tracer To Study 
Ventilation Systems in Mines. BuMines RI 
7916, 1974, 22 pp. 

6. Vinson, R. P., J. C. Volkwein, and 
E. D. Thimons. SF 6 Tracer Gas Tests of 
Bagging-Machine Hood Enclosures. BuMines 
RI 8527, 1981, 10 pp. 

7. Mody, V., W. Harris, R. Jukhete, 

D. Goldheim. Conveyor Belt Dust Control. 
Final report on BuMines Contract HOI 13007 
with Martin Marietta Lab. , February 1984, 
150 pp.; Richard Wilson, Twin Cities 
Research Center, BuMines, Minneapolis, 
MN. 

8. U.S. Bureau of Mines. Air Curtain 
Provides Dust Protection. Technol. News, 
No. 21, Jan. 1976, 2 pp. 

9. Volkwein, J. C. , S. J. Page, and 

E. D. Thimons. Canopy-Air Curtain Dust 
Reductions on a Gathering-Arm Loader. 
BuMines RI 8603, 1982, 9 pp. 

10. Volkwein, J. C. Dust Control in 
Bagging Operations. Paper in Industrial 



Hygiene for Mining and Tunneling — Pro- 
ceedings of a Topical Symposium (Denver, 
CO., Nov. 6-7, 1978). ACGIH, Cincinnati, 
OH, 1979, pp. 51-57. 

11. Muldoon, T. Private communica- 
tion, 1983; available upon request from 
T. Muldoon, Foster-Miller Inc., Waltham, 
MA, or J. C. Volkwein, BuMines, Pitts- 
burgh, PA. 

12. U.S. Bureau of Mines. Dust Knock- 
down Performance of Water Spray Nozzles. 
Technol. News, No. 150, July 1982, 2 pp. 

13. Hiltz, R. H. , and J. V. Fried. 
Using High Expansion Foam To Control Res- 
pirable Dust. Min. Congr. J., May 1973, 
pp. 54-60. 

14. Seibel, R. J. Dust Control at a 
Transfer Point Using Foam on Water 
Sprays. BuMines TPR 97, May 1976, 12 pp. 



15. Volkwein, J. C, A. B. Cecala, and 
E. D. Thimons. Adding Foam To Control 
Dust in Minerals Processing. BuMines RI 
8808, 1983, 11 pp. 

16. Cecala, A. B. Private communica- 
tion 1984; available upon request from 
A. B. Cecala, BuMines, Pittsburgh, PA. 

17. Cheng, L. , E. E. Emmerling, and 
T. F. Tomb. Collection of Airborne Coal 
Dust by Steam. BuMines RI 7819, 1974, 
13 pp. 

18. Volkwein, J. C. , R. P. Vinson, and 
E. D. Thimons. Effectiveness of Three 
Water Spray Methods Used To Control Dust 
During Bagging. BuMines RI 8614, 1982, 
9 PP. 

19. McCoy, J., J. Melcher, J. Valen- 
tine, D. Monaghen, T. Muldoon, and J. 
Kelly. Evaluation of Charged Water 
Sprays for Dust Control. BuMines OFR 98- 
83, 1983, 150 pp.; NTIS PB 83-210-476. 



*U.S. CPO: 1985-505-019/20,006 



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